The invention generally relates to sandwich structures and more particularly to superplastically formed, diffusion bonded, orthogonally corrugated core sandwich panels.
Ever since the first successful application of the sandwich structures in the British all-wood "Mosquito" aircraft during World War II, sandwich structures have attained widespread usage, particularly in the aerospace industry, in the wings, wall panels, webs of beams, empennage structures and the like.
The common sandwich structure usually consists of two relatively thin, high strength face sheets separated by and bonded to a relatively thick, low density, low strength core. This type of composite structure generally is characterized by low-mass as well as high flexural stiffness characteristics.
The most extensively used sandwich structure found in the aerospace technology is the so-called honeycomb core sandwich. Usually, this structure includes a core of aluminum or titanium conforming to a honeycomb cell generatrix disposed between rigid face sheets. In such structures, the honeycomb cell generatrix is arranged perpendicularly to the face sheets and, therefore, the bonding between the honeycomb core and the face sheets can only be achieved by line-contact. As is well known, honeycomb structures sometimes fail to meet existing needs because the line-contact bonding, established between the honeycomb cell cross section and the face sheets, can and frequently does lose bonding integrity, due in part to the effects of corrosion or the like.
In recent years, a revolutionary new process known generally as superplastic forming with concurrent diffusion bonding has emerged in the fabrication of sandwich structures. This process utilizes two inherent phenomena which tend to occur concurrently in titanium alloys. The first phenomenon is the ability of a material, such as a titanium alloy, to undergo large, up to 1000% strain, plastic deformations at high temperatures without localized thinning, or necking. This phenomenon often is referred to as superplasticity. The second phenomenon relates to the capability of being joined under pressures at elevated temperatures, without melting or the use of bonding agents, herein referred to simply as diffusion bonding.
By employing the above-mentioned phenomena, sandwich structures frequently are fabricated by a diffusion-bonding of at least three superplastic alloy sheets, at selected areas, and then superplastically expanding or separating the face sheets by internal pressurization. Pressurization is achieved through a use of inert gases, such as argon, introduced into the core cavity in order to impart a final configuration to the structure. This known process tends to eliminate the use of bonding agents and facilitates surface-contact bonding, instead of line-contact bonding as occurs in the formation of the aforementioned honeycomb sandwich structures.
Through the new technique, aforedescribed, it now has become possible to provide a relatively large number of new shapes and symmetries for sandwich cores. These new shapes and symmetries include the so-called truss cores, dimpled cores, sine-wave cores, egg-box-shaped cores, truncated cores, hollow truncated square hexogonal pyramids arrayed in square or hexogonal patterns. The truss core has a relatively high flexural stiffness in the direction of the corrugation but tend to be characterized by a very low flexural stiffness in directions transverse to the axes of corrugation. Other types of the cores aforementioned also tend to be characterized by weak bending stiffness. For example, dimpled core structures tend to exhibit very low out-of-plane bending stiffness in any in-plane direction.
Typifying the methods and structures of the prior art are those disclosed in U.S. Pat. Nos. 2,766,514; 3,369,288; 3,924,793; 3,927,037; and 4,087,037. It is believed that U.S. Pat. No. 3,927,037 probably contains teachings more pertinent to the instant invention than the teachings of the remaining listed patents, since this patent discloses the use of four panel members interconnected for forming a metallic sandwich structure. However, it is believed to be apparent that the structure therein disclosed lacks the structural strength characteristics which tend to characterize the sandwich structure hereinafter more fully described.
It should now be apparent that there persists a need for an expanded metallic cellular structure which tends to overcome the aforementioned difficulties and disadvantages, without impairing the utility thereof.
It is therefore the purpose of the instant invention to provide an improved core structure having enhanced bending stiffness, in any in-plane direction, and one which can readily and economically be fabricated utilizing the known characteristics of superplastic alloys.